Pulsed-neutron formation evaluation tools interrogate the formation surrounding the borehole with high-energy neutrons produced by a neutron generator forming part of the tool. Because of interaction by the neutrons with elements of the tool, with the borehole, and with the formation, gamma radiation is created, which is then measured by gamma radiation sensors that also form part of the tool. Measurement data captured by the gamma radiation sensors can be processed to derive information about the properties of the borehole and the subsurface formations.
The energetic neutrons are commonly produced by a deuterium-tritium (D/T) fusion reaction caused, for example, by generating an ion beam and accelerating it into collision with a target loaded with target molecules or atoms.
Some formation properties are evaluated by means of gamma radiation at lower energies than that produced by such pulsed neutron generator tubes. Porosity logging tools, for example, typically make use of AmBe chemical neutron sources.